We set out to answer a question that does not seem to have a good answer: “What happens to bourbon when stored in different real world conditions and fill levels over time?”

General consensus is that a full, sealed bottle of bourbon stored at a moderate to cool temperature in the dark can last for years, decades, or maybe even centuries. But reality is, we open the majority of our bourbon and drink it down over time - those bottles that we bring out only for special occasions may be open for a while. We showcase our bourbon on shelves in rooms that have natural sunlight. The temperature in our homes fluctuates - not everyone has air conditioning or a cool basement.

It is a common belief that the more air inside the bottle, the faster the rate of oxidation - this is obvious. However, how much air, what storage conditions, and at what rate the bourbon will oxidize is not quite as clear.

We’ve heard from multiple sources that once you get down to about about ⅓ full (some say ½ while others say ¼), it’s time to invite some friends over and finish the bottle off because it’s going to spoil fairly quickly at that point anyway. We’ll take a closer look at this and try to determine more specifically what fill level, storage conditions, and elapsed time have on the bourbon.

There are scientific explanations of what happens at a molecular level in certain conditions as well. There are experiments somewhat like this one. There are great articles and discussion threads pulling from extensive personal experience. Despite these excellent resources, everything we’ve read fails to combine solid supporting evidence AND specific recommendations combined with specific time intervals and varying real-world storage conditions. [Check out the references at the end of this article for some additional information relative to this topic.

‍To that end, we decided to conduct our own experiment with the goal of answering the following basic questions:

1) At what point is there too much air relative to liquid in the bottle? In other words, at what point do you invite your friends over to finish it off, transition into smaller bottles, or apply an inert gas filler like Private Preserve?‍

2) What effect does temperature have on bourbon over time? Is room temperature best, or should you consider refrigeration for long term storage?

3) What effect does opening and closing a bottle (“drinking it down” and introducing oxygen repeatedly) have over time relative to the various storage conditions or simply keeping a bottle sealed. ‍

4) What effect does sunlight have?‍

5) If your bottle is exposed to sunlight, is there any noticeable benefit to an amber glass vs. clear glass container? ‍

6) At what specific time intervals do changes start to take place given a range of real-world storage conditions?

The bourbon chosen was a brand new 1.75L bottle of W. L. Weller Special Reserve. We consider this to have a straightforward and traditional flavor profile, not too robust, but not too mild either. At 90 proof, it is on the more delicate side of the proof spectrum, so we anticipate more noticeable changes than a higher proof bourbon might have.

‍3 Different Storage Conditions:

For each of the storage conditions, we used a total of 10 2-oz clear sample bottles at various fill levels. For the direct sunlight location we added a 2-oz amber sample bottle for a total of 11 bottles in that specific location - a grand total of 31 bottles. We believe the sample bottles will replicate the concept of whole bottles, as the total volume of bourbon relative to air should be scalable at the molecular level. Furthermore, there’s no way we were buying 31 bottles of Weller Special Reserve for this experiment!

Samples from each storage condition at different fill levels will be tasted at 6 month, 12 month, and 24 month intervals. They will be compared to one another for smell, taste, complexity, and overall qualitative experience. Although there will not be a specific reference point, the full-fill samples stored in the dark closet at room temperature will be considered our best reference point, as this would generally be considered an ideal storage condition with little expectation of change over a 24 month period. Although we considered buying a new bottle of Weller for each tasting as a comparison, we believe the slight fluctuation in batches could skew the results.

‍Other Important Notes:

Refrigerator stored bottles will be allowed to acclimate to room temperature for at least 24 hours before tasting.

The bottles starting on the left are full fill to start. At each interval, they will be opened and ⅓ of the bottle will be tasted. They will then be resealed and ⅓ will be tasted at the next interval. By drinking ⅓ at each interval, they will be completely consumed by the end of the experiment. This situation is intended to represent drinking down a bottle over a 2 year period of time.

The farthest bottle on left in the direct sunlight picture is the amber bottle. The remainder of the bottles are clear glass.‍

‍What this experiment will not tell us:‍

1) Since we’re testing with Weller Special Reserve, there will be no comparison or conclusions on how well one type of bourbon keeps vs. another when subjected to various storage conditions.

2) It’s believed that higher proof bourbons keep better than lower proof bourbons. This experiment will not test that theory, but we did specifically choose a lower proof bourbon on the assumption that the effects of oxidation may be more easily identified.

3) Since we’re using sample bottles that have a tight seal, this experiment will not tell us the impact of a failed cork or air exposure due to an unsealed bottle. However, based on our personal experience a failed cork/bad seal causes bourbon to go bad fairly quickly

4) We will not be testing one type of cork vs. another, a cork vs. screw cap, or how the shape of a bottle might impact the bourbon over time

5) We will not be testing a situation where a bottle is opened and closed excessively. We would assume that if you’re doing this, you’re either drinking the bottle so quickly it doesn’t matter or you’re taking it out to nose it all the time without actually drinking it. We do not recommend the latter, and would suggest that you pull off a small sample to nose if you find yourself doing this.

‍Resources & References:‍

We’ve scoured the web to find any additional information on this topic. We’ve selected a handful of references with any kind of substantial information that give insight on this topic and linked to them below. This is by no means a complete list, but a pool of resources we found helpful. If you have any additional resources you think should be added to the list, please post in the Comments section or contact us.

A similar experiment performed by SKU with different fill levels, one storage condition (his closet), a single malt scotch whiskey (Longmorn 16), and a single tasting performed at 2 years and 2 months. This is quite possibly the best supporting "Real world" evidence we’ve seen so far.